Concrete Calculator
Cubic yards and bags for slabs, footings and posts.
Reviewed by the Calculation Machine team · Updated 24 October 2026 · How we check our maths
Your values
ⓘ Live results
- Concrete needed
- 2.72 cubic yards
- Cubic feet
- 73.3
- 80 lb bags
- 123
- 60 lb bags
- 163
- Approximate weight
- 11,000 lb
Quick answer
To calculate concrete, multiply length × width × thickness in feet to get cubic feet, then divide by 27 for cubic yards. Convert thickness from inches by dividing by 12. A 10 × 10 ft slab 4 in thick is 10 × 10 × 0.333 = 33.3 ft³, or 1.36 cubic yards with 10% waste – about 62 bags of 80 lb premix.
What the Concrete calculator does
This calculator gives the volume of concrete for any rectangular pour – patio and driveway slabs, shed bases, continuous wall footings and pads – in cubic yards and cubic feet, plus the number of 80 lb and 60 lb bags and the approximate weight. A waste allowance (10% by default) covers spillage, uneven subgrade and over-excavation.
Use cubic yards when ordering ready-mix from a plant and the bag count when mixing yourself. As a rule of thumb, bagged concrete makes sense up to about 1 cubic yard; beyond that, the 45-plus bags per yard and the mixing time usually make a ready-mix delivery cheaper and give a more consistent slab.
For round post holes use the fence post concrete calculator; for block walls use the concrete block calculator. To price the job, the concrete slab cost calculator adds materials and labor.
The formula
Volume (ft³) = L × W × (T ÷ 12) × (1 + waste ÷ 100) Cubic yards = ft³ ÷ 27 Bags = ft³ ÷ bag yield (0.60 ft³ per 80 lb, 0.45 ft³ per 60 lb), rounded up
| Symbol | Meaning | Unit |
|---|---|---|
| L | Length of the pour | ft |
| W | Width of the pour (for a footing, its width) | ft |
| T | Thickness of a slab or depth of a footing | in |
| waste | Extra allowance for spillage and uneven base | % |
| 27 | Cubic feet in one cubic yard (3 × 3 × 3) | ft³/yd³ |
Bag yields are the figures printed by major US premix makers (about 0.60 ft³ for an 80 lb bag and 0.45 ft³ for a 60 lb bag). Weight assumes normal-weight concrete at about 150 lb/ft³, roughly 4,000 lb per cubic yard. For metric work, 1 cubic yard = 0.765 m³. The formula is for rectangular shapes only – split L-shaped slabs into rectangles and add them, and use a cylinder volume for round piers.
Worked examples
Shed base slab
A 10 × 10 ft slab, 4 in thick, for a garden shed.
Inputs
- Length10 ft
- Width10 ft
- Thickness4 in
- Waste allowance10 %
Result
- Concrete needed1.36 cubic yards
- Cubic feet36.7
- 80 lb bags62
- 60 lb bags82
- Approximate weight5,500 lb
About 1.36 cubic yards or 62 bags of 80 lb mix. That is on the edge where a short-load ready-mix delivery is worth pricing against roughly 2.5 tons of bags.
Two-car garage floor
A 24 × 24 ft garage slab, 6 in thick, with an 8% allowance on a well-compacted base.
Inputs
- Length24 ft
- Width24 ft
- Thickness6 in
- Waste allowance8 %
Result
- Concrete needed11.52 cubic yards
- Cubic feet311
- 80 lb bags519
- 60 lb bags692
- Approximate weight46,656 lb
11.52 cubic yards is a full truck – order ready-mix. Ask the supplier whether to round up to the nearest half or quarter yard; running short mid-pour leaves a cold joint.
Continuous wall footing
A 40 ft run of footing, 16 in (1.33 ft) wide and 12 in deep, for a garden wall.
Inputs
- Length40 ft
- Width1.33 ft
- Thickness12 in
- Waste allowance10 %
Result
- Concrete needed2.17 cubic yards
- Cubic feet58.5
- 80 lb bags98
- 60 lb bags131
- Approximate weight8,778 lb
About 2.17 cubic yards. Trenches are rarely perfectly square, so keep the 10% allowance or measure the depth at several points and use the average.
Typical thickness by project
Thickness drives volume more than anything else: going from 4 in to 6 in adds 50% more concrete. These are common US residential figures; local building codes, soil conditions and loads can require more, so check with your building department for structural work.
| Project | Typical thickness | Notes |
|---|---|---|
| Walkway, patio | 4 in (100 mm) | On 4 in of compacted gravel |
| Shed or hot tub base | 4–6 in | Thicken edges for heavy loads |
| Driveway (cars) | 4–5 in | 5–6 in for trucks or RVs |
| Garage floor | 4–6 in | Reinforce with mesh or rebar |
| Wall footing | 8–12 in deep | Width often twice the wall thickness |
Footings and shed foundations
- Continuous footings: enter the total run as length, the footing width as width and its depth as thickness. Convert width in inches to feet first – 16 in is 1.33 ft, 20 in is 1.67 ft.
- Square pad footings: work out one pad (for example 2 × 2 ft, 12 in deep) and multiply the bags by the number of pads.
- Shed bases: a slab 4 in thick is typical for sheds; size it a few inches larger than the shed floor on each side.
- Frost depth: footings must reach below the local frost line, which ranges from a few inches in the southern US to 4 ft or more in the far north. Your building department publishes the local figure.
For the gravel layer under a slab, use the gravel calculator.
Bags versus ready-mix
One cubic yard takes 45 bags of 80 lb or 60 bags of 60 lb mix, weighing about 3,600 lb in total. Mixing that by hand or with a small mixer is a long day for two people. Ready-mix plants usually set a minimum order or charge a short-load fee below a few yards, so get quotes for both if your pour is between 1 and 3 cubic yards. Concrete must be placed and finished in one continuous pour for a slab without cold joints.
How to use the Concrete Calculator
- 1Measure the length and width of the form in feet. Convert inches to feet by dividing by 12 (6 in = 0.5 ft).
- 2Enter the slab thickness or footing depth in inches.
- 3Keep a 5–10% waste allowance; use the higher figure for uneven ground or hand-dug trenches.
- 4Read cubic yards for a ready-mix order, or the bag count for premix, and round up.
Frequently asked questions
How many bags of concrete are in a cubic yard?
About 45 bags of 80 lb mix or 60 bags of 60 lb mix, since each yields roughly 0.60 or 0.45 cubic feet and a cubic yard is 27 cubic feet.
How much concrete do I need for a 10×10 slab?
At 4 in thick, a 10 × 10 ft slab needs 1.23 cubic yards before waste, or about 1.36 cubic yards with 10% added. At 6 in thick it needs half as much again.
How much does a cubic yard of concrete weigh?
Roughly 4,000 lb (about 2 tons, or 2,400 kg per cubic meter) for normal-weight concrete. Check that your vehicle or trailer can carry bags before buying in bulk.
How much extra concrete should I order?
Add 5–10% for slabs on a level, compacted base and up to 15% for trenches or irregular excavation. Running short mid-pour is far more costly than a small surplus.
Do I need rebar in a concrete slab?
Driveways, garage floors and structural slabs usually need wire mesh or rebar; small patios and walkways often use control joints only. The rebar calculator works out bar lengths for a grid.
References
- ACI 332 – Residential Code Requirements for Structural Concrete — American Concrete Institute
- International Residential Code (IRC), Chapter 4: Foundations — International Code Council
- Design and Control of Concrete Mixtures — Portland Cement Association
About this calculator
- Built and reviewed by the Calculation Machine team · How we build and test calculators
- Last reviewed: October 24, 2026
- Report an error